1. Field of the Invention
The present invention relates to an anti-lock brake control device and a brake control device which are capable of performing a selection-low control.
2. Discussion of the Related Art
Heretofore, as one type of anti-lock brake control devices in this kind, there has been known one which is provided with wheel rotational speed detection means for detecting respective rotational speeds of left and right wheels in the same system and fluid pressure regulating means for independently regulating brake fluid pressures in wheel cylinders which respectively restrict rotations of the left and right wheels. During braking of the vehicle, the fluid pressure regulating means is controlled in any of a pressure increasing mode, a pressure reducing mode and a pressure retention mode in dependence on the rotational speeds of the wheels, and the left and right wheels in the same system are placed under a selection-low control so that the vehicle stability can be secured (refer to Patent Document 1).
As another type, there has also been known one in which front and rear wheels are placed under the selection-low control (refer to Patent Document 2). Specifically, in an anti-lock brake control device described in Patent Document 2, brake fluid pressures for front left and right wheels are controlled independently, and brake fluid pressures for rear left and right wheels are controlled coordinately. With respect to the front wheels, the rear wheels are controlled under the selection-low control in ordinary traveling, but in traveling on a split road with brake being applied, the rear wheels are controlled under a selection-high control and a control threshold value for use in controlling the brake fluid pressures for the rear wheels is compensated to lessen the lock of the rear wheels (S44). The anti-lock brake control device is enabled to be modified so that compensation is likewise made also to lessen the lock during turn traveling on a split road or during turn traveling. The anti-lock brake control device is further enabled to be modified so that the rear wheels are always controlled under the selection-low control and so that the control threshold valve for use in controlling the brake fluid pressures for the rear wheels can be compensated to make the lock stronger in traveling on a split road or in making a turn.                [Patent Document 1] Japanese unexamined, published patent application No. 01-269655 (pp. 3-6 and 11, FIG. 3)        [Patent Document 2] Japanese unexamined, published patent application No. 08-099622 (pp. 5-94, FIGS. 7 and 14)        
In the last mentioned anti-lock brake control device, a problem arises in that a sufficient vehicle braking performance cannot be secured due to a difference between braking effects on the left and right wheels. That is, where because of the difference, the selection-low control is performed on the left and right wheels in the same system during braking the vehicle which is traveling on an even road surface, an anti-lock brake control is performed on one wheel which is stronger in the braking effect thereon, and the other wheel which is weaker in the braking effect thereon cannot exercise a sufficient braking performance thereof.
Further, where the left and right wheels have a difference between braking effects thereon, another problem also arises in that the vehicle stability is deteriorated though the vehicle braking performance can be secured sufficiently. That is, when the selection-low control is performed on the left and right wheels in the same system during braking the vehicle traveling on a split road surface in which case one wheel stronger in the braking effect thereon is on a higher-μ side (i.e., on a side with a higher friction coefficient), an anti-lock brake control is performed for the other wheel at the opposite side, so that the wheel stronger in the braking effect thereon would exercise a more braking performance than as needed.